1. Field
The present invention relates to a fuel cell system and a fuel supply method thereof, and, more particularly, to a fuel cell system and a fuel supply method thereof, which are capable of reducing pulsation of fuel supplied to a reformer and a burner, thereby stably supplying the fuel, and maintaining a desired ratio of the components of modifying gas supplied to a stack.
2. Description of the Related Art
Generally, a fuel cell is an electricity generating device to directly convert chemical energy of hydrogen and oxygen contained in hydrocarbon-based fuel such as methanol, ethanol, or natural gas into electrical energy in accordance with an electrochemical reaction. FIG. 1 is a schematic view to explain the electricity generating principle of such a fuel cell. When air containing oxygen is supplied to a cathode 1, and fuel containing hydrogen is supplied to an anode 3, a reaction reverse to electrolysis of water occurs through an electrolyte membrane 2, so that electricity is generated. Typically, the electricity generated from such a unit cell 4, has a voltage too low to enable the electricity to be useful. To this end, several unit cells 4 are connected in series in the form of a stack.
To produce hydrogen to be supplied to such a fuel cell stack (hereinafter, simply referred to as a “stack”), a fuel treating unit is used. The fuel treating unit includes a reformer to modify fuel (for example, city gas for domestic use), and thus, to produce high-quality hydrogen, and a burner to heat the reformer such that the reaction temperature of the reformer is maintained at an appropriate temperature. Fuel to be modified for production of high-quality hydrogen (hereinafter, referred to as “reformer fuel”) is supplied to the reformer, together with water (deionized water (DI-water)). Fuel to be burnt for adjustment of the reformer temperature (hereinafter, referred to as “burner fuel”) is supplied to the burner, together with air containing oxygen. Such reformer fuel and burner fuel have considerable influence on the production of high-quality hydrogen. That is, the operation performance of the fuel cell system is considerably influenced by a steam to carbon ratio (S/C) control to control the ratio of the reformer fuel to water, at which the reformer fuel and water are supplied to the reformer, and thus, to control the carbon monoxide concentration determining the performance of the stack, and a lamda (the ratio of burner fuel to air) control to control the ratio of the burner fuel to air, at which the burner fuel and air are supplied to the burner, and thus, to enable the reformer fuel to efficiently react in the reformer while maintaining a flame at the burner.
In the case of a domestic fuel cell system, fuel, namely, city gas, is supplied at a pressure of about 2 to 3 Kpa. For this reason, the domestic fuel cell system uses a fuel pump or the like, taking into consideration a pressure loss occurring in the system and pulsation occurring during the reaction between the fuel treating unit and the stack. The fuel pump is used together with a blower, a valve, etc., in order to achieve the supply of fuel to the interior of the fuel cell system and other operations. These devices are referred to as “balance-of-plant (BOP)” units. The specification of each BOP unit is determined, taking into consideration the internal system pressure loss and pulsation. However, such pressure loss and pulsation phenomena are continuously varied, are not in a static state, but rather are in a dynamic state. For this reason, in the case of a particular BOP unit, for example, the fuel pump, it is impossible to determine the stable and efficient specification of the fuel pump. The operation of the fuel pump may be influenced by the internal/external system pressure loss (pressurization) and pulsation. In particular, in the domestic fuel cell system, considerable pulsation may occur due to a variation in fuel flow rate because the fuel supply pressure may be lowered to at least 1 Kpa. In this case, a flame may fail. Also, the concentration of carbon monoxide may be unstable. As a result, an imperfect operation condition may occur. Furthermore, when it is necessary to rapidly start up the fuel cell system, a burner fuel pump may be additionally used to increase the flow rate of the burner fuel. However, this is non-economical in terms of costs. Under such circumstances, it is important to provide an economical and stable fuel supply system for the fuel cell system.